1. Field of the Invention
This invention relates to particle sensors and particularly to particle sensors that use optical scanning and detection of scattered light.
2. Description of Related Art
Particle contaminants in semiconductor fabrication equipment such as plasma etch and vapor deposition chambers can deposit on semiconductor wafer surfaces and cause manufacturing defects that reduce the yield of operable devices. Great care is therefore taken to avoid particle contamination in such equipment. Even so, processing a batch of devices can create particles that cause defects in subsequent batches, and physical maintenance (or cleaning) of the equipment is periodically performed to reduce or eliminate these contaminants. Ideally, physical maintenance is only performed when needed to prevent the defect rate from reaching an undesirable level. If physical maintenance is too frequent, down time increases which reduces the total number of devices produced.
Physical maintenance can be scheduled according to the number of batches processed where the number of batches between scheduled maintenance is fixed and corresponds to a particular probability of the defect rate being unacceptably high. However, with this approach, physical maintenance at fixed intervals could be performed before cleaning is required which unnecessarily increases down time. At other times the physical maintenance may be too infrequent resulting in batches having unacceptable defect rates. Alternatively, physical maintenance may be initiated when device testing or wafer surface evaluation identifies an increased defect rate. However, several batches with high defect rates may be produced before test results are known. Another technique initiates physical maintenance when particle counts in a pump-line of the equipment become high. The primary disadvantage of this technique is that pump-line particle counts may not correlate with the defect rates at the wafer surface. This could potentially lead to inaccurate indications of physical maintenance requirements.
An improved technique would detect particles near the surface of a wafer during processing. This necessitates a particle sensor that can cope with the environment inside a reaction chamber and not disturb the manufacturing process. The particle sensor must detect particles less than a micrometer in diameter in close proximity to a wafer, possibly in the presence of a plasma, and should sample a relatively large volume near the surface of the wafer because the particle distribution may not be uniform across the surface of a wafer during processing and because the particle concentration is typically low near the wafer surface. Additionally, to provide maximum utility to the user, the sensor should provide particle size, particle velocity, and particle location information for each particle detected. Before the present invention, a particle sensor which met all of these requirements for in situ detection of particles during semiconductor manufacture was not available.